Re: [PATCH v6 bpf-next 6/7] bpf: introduce bpf_prog_pack allocator

[Alexei Starovoitov]

On Fri, Jan 21, 2022 at 11:49 AM Song Liu <song@xxxxxxxxxx> wrote:
>
> +static struct bpf_binary_header *
> +__bpf_jit_binary_alloc(unsigned int proglen, u8 **image_ptr,
> +                      unsigned int alignment,
> +                      bpf_jit_fill_hole_t bpf_fill_ill_insns,
> +                      u32 round_up_to)
> +{
> +       struct bpf_binary_header *hdr;
> +       u32 size, hole, start;
> +
> +       WARN_ON_ONCE(!is_power_of_2(alignment) ||
> +                    alignment > BPF_IMAGE_ALIGNMENT);
> +
> +       /* Most of BPF filters are really small, but if some of them
> +        * fill a page, allow at least 128 extra bytes to insert a
> +        * random section of illegal instructions.
> +        */
> +       size = round_up(proglen + sizeof(*hdr) + 128, round_up_to);
> +
> +       if (bpf_jit_charge_modmem(size))
> +               return NULL;
> +       hdr = bpf_jit_alloc_exec(size);
> +       if (!hdr) {
> +               bpf_jit_uncharge_modmem(size);
> +               return NULL;
> +       }
> +
> +       /* Fill space with illegal/arch-dep instructions. */
> +       bpf_fill_ill_insns(hdr, size);
> +
> +       hdr->size = size;
> +       hole = min_t(unsigned int, size - (proglen + sizeof(*hdr)),
> +                    PAGE_SIZE - sizeof(*hdr));

It probably should be 'round_up_to' instead of PAGE_SIZE ?

> +       start = (get_random_int() % hole) & ~(alignment - 1);
> +
> +       /* Leave a random number of instructions before BPF code. */
> +       *image_ptr = &hdr->image[start];
> +
> +       return hdr;
> +}
> +
>  struct bpf_binary_header *
>  bpf_jit_binary_alloc(unsigned int proglen, u8 **image_ptr,
>                      unsigned int alignment,
>                      bpf_jit_fill_hole_t bpf_fill_ill_insns)
> +{
> +       return __bpf_jit_binary_alloc(proglen, image_ptr, alignment,
> +                                     bpf_fill_ill_insns, PAGE_SIZE);
> +}
> +
> +struct bpf_binary_header *
> +bpf_jit_binary_alloc_pack(unsigned int proglen, u8 **image_ptr,
> +                         unsigned int alignment,
> +                         bpf_jit_fill_hole_t bpf_fill_ill_insns)
>  {
>         struct bpf_binary_header *hdr;
>         u32 size, hole, start;
> @@ -875,11 +1034,16 @@ bpf_jit_binary_alloc(unsigned int proglen, u8 **image_ptr,
>          * fill a page, allow at least 128 extra bytes to insert a
>          * random section of illegal instructions.
>          */
> -       size = round_up(proglen + sizeof(*hdr) + 128, PAGE_SIZE);
> +       size = round_up(proglen + sizeof(*hdr) + 128, BPF_PROG_CHUNK_SIZE);
> +
> +       /* for too big program, use __bpf_jit_binary_alloc. */
> +       if (size > BPF_PROG_MAX_PACK_PROG_SIZE)
> +               return __bpf_jit_binary_alloc(proglen, image_ptr, alignment,
> +                                             bpf_fill_ill_insns, PAGE_SIZE);
>
>         if (bpf_jit_charge_modmem(size))
>                 return NULL;
> -       hdr = bpf_jit_alloc_exec(size);
> +       hdr = bpf_prog_pack_alloc(size);
>         if (!hdr) {
>                 bpf_jit_uncharge_modmem(size);
>                 return NULL;
> @@ -888,9 +1052,8 @@ bpf_jit_binary_alloc(unsigned int proglen, u8 **image_ptr,
>         /* Fill space with illegal/arch-dep instructions. */
>         bpf_fill_ill_insns(hdr, size);
>
> -       hdr->size = size;

I'm missing where it's assigned.
Looks like hdr->size stays zero, so free is never performed?

>         hole = min_t(unsigned int, size - (proglen + sizeof(*hdr)),
> -                    PAGE_SIZE - sizeof(*hdr));
> +                    BPF_PROG_CHUNK_SIZE - sizeof(*hdr));

Before this change size - (proglen + sizeof(*hdr)) would
be at least 128 and potentially bigger than PAGE_SIZE
when extra 128 crossed page boundary.
Hence min() was necessary with the 2nd arg being PAGE_SIZE - sizeof(*hdr).

With new code size - (proglen + sizeof(*hdr)) would
be between 128 and 128+64
while BPF_PROG_CHUNK_SIZE - sizeof(*hdr) is a constant less than 64.
What is the point of min() ?